Impact of Hybrid Distributed Generator on Transient ...ijmtst.com/documents/NCCEESES15.pdf · is done in a user friendly MATLAB/Simulink environment. ... behind the use of hybrid

Volume 2 | Special Issue 01 | October 2016| ISSN: 2455-3778| www.ijmtst.com 91

Proceedings of National Conference on Computing, Electrical, Electronics and Sustainable Energy Systems

Impact of Hybrid Distributed Generator on Transient Stability of Power System

V. Sujatha1 | M. Umarani2

1PG Scholar, Department of EEE, Godavari Institute of Engineering and Technology, Rajahmundry, Andhra Pradesh, India. 2Assistant Professor, Department of EEE, Godavari Institute of Engineering and Technology, Rajahmundry, Andhra Pradesh, India.

ABSTRACT

Due to increasing integration of new technologies into the grid such as hybrid electric vehicles,

distributed generations, power electronic interface circuits, advanced controllers etc., the present power

system network is now more complex than in the past. Consequently, the recent rate of blackouts

recorded in some parts of the world indicates that the power system is stressed. The real time/online

monitoring and prediction of stability limit is needed to prevent future blackouts.

The aggravated increase in energy demand has posed a serious problem for the power system’s

stability and reliability, and hence has become of major concern. The shortcomings of conventional

source of energy have paved way for renewable energy sources. The latter can form a part of a stand

alone system or grid connected system. A single renewable source of energy When integrated with

other sources of energy it is termed as hybrid system. This thesis deals with PV, Wind, Hydro system.

In this thesis an active power control strategy has been developed such that when the wind alone

is not able to meet the energy demand, without compromising the frequency a transition occurs to wind

diesel mode so that the energy demand is met. The mathematical model considered uses a STATCOM

to meet the reactive power need upon sudden step change in power. The performance and the analysis

is done in a user friendly MATLAB/Simulink environment.

KEYWORDS: Transient Stability, Distributed Generator, Lithium Ion Battery, Terminal Voltage and

Rotor Speed Deviation.

Copyright © 2016 International Journal for Modern Trends in Science and Technology

All rights reserved.

I. INTRODUCTION

Distributed generation is getting a lot of

focus in recent years due to various energy and

environmental concerns. Unlike the

conventional generators, the distributed

generators use both non-renewable and

renewable sources of energy to generate

electricity. Major technical and economic issues

arise when these DG‟s are interconnected with

the electric grid. The technical issues include

stability, power quality, protection issues and

voltage fluctuations. It has to be noted that

certain renewable generators such as wind and

solar do not produce power at a constant rate

since they are dependent on the natural forces.

This makes the use of storage devices very

essential.

There has been increased point up in the

decades on the concept of smoothing

intermittent output of distributed generation

(DG) using energy storage [1]. DGs can be

defined as the concept of connecting generating

units of small sizes, between several kW to a

few MW. The primary source of energy for these

generators can be the traditional non -

renewable sources such as gas or the

renewable sources such as wind, solar, hydro,

and biomass [3]. These generators are

connected either to the medium voltage or low

voltage sections of the electric grid. Most often



they are connected near the load centers or the

low voltage networks.

In case of certain renewable technologies,

such as wind turbines and solar panels, the

output power depends upon the availability of

renewable resource and therefore may not

always be constant. In such cases, in order to

augment the DGs during low power periods,

energy storage devices are used [3]. Other than

wind and solar, storage can help in smoothing

the power in conjunction with biomass

especially for rural combined heating and

power applications. These devices store energy

during periods of high power or low demand

and use the stored energy to supply the excess

loads during periods of low power. Different

types of energy storage devices that are used in

a distributed environment include batteries,

ultra capacitors, flywheels, fuel cells and

superconducting magnetic energy storage [4]-

[7]. In addition to supporting the DGs during

peak demand, the storage devices may also

help in improving the overall stability of the

entire system. These energy storage devices are

connected to the electric grid by means of

suitable power conversion devices [8]-[10].

Certain DG techniques such as wind and

solar, do not output constant power at all

times. In such cases, storage of power for later

use becomes essential. The energy storage

devices store power from the DG‟s during times

of normal output and low loads. This stored

energy can be used at time when there is no

DG output or during periods of high demands.

The storage device that needs to be used

depends upon the application, amount of

storage needed and the duration of storage.

Most of the storage devices are DC, so we need

power electronic interfaces to connect them to

the AC system.

DG‟s can operate in two modes namely

stand-alone and grid connected. The grid

connected mode has drawn a lot of attention

and analysis recently, due to its impacts on the

grid. Though the DG has a lot of advantages, it

also has some negative impacts especially when

connected to the grid.

The most critical technical issue is the

stability of the system. There have been a

number of studies on the both the steady state

and the transient stability impacts of DG on

systems. As mentioned earlier, energy storage

devices also have significant impact on the

system. Based on the indicators and approach

selected in the references, a procedure to

analyze the stability has been obtained. This

work also involves modeling of two energy

storage devices, battery and ultra-capacitor.

II. DISTRIBUTED GENERATION AND HYBRID

DISTRIBUTION GENERATION

The definition of distribution generation

(DG) in the literature can be stated as follows:

A small capacity power plant based on either

combustion-based technologies, such as

reciprocating engines and turbines, or non-

combustion based technologies such as fuel

cells, photovoltaic, wind turbines, located on or

near end-users and are characterized as

renewables or cogeneration non-dispatched

sources

HDG is expected to form an active part of

future power system network in order to meet

the future increasing demand for energy. The

inherent potential to provide higher quality

power, minimize power loss and produce more

reliable power to consumers than a system

based on a single source is the motivation

behind the use of hybrid power generation. The

objective of the integration is to capitalize on

the strengths of both conventional and

renewable energy sources, both cogeneration

and non-cogeneration types. HDG can either be

grid-connected or standalone system,

renewable or nonrenewable system. HDG with

one or more renewable (stochastic) or non-

stochastic energy sources interact with the

existing grid during import and export of power

generation. This interaction contributes

different level of fault current, reactive power,

different inertia, therefore making the system

vulnerable to different instabilities compare to

single energy source. Presently, the promising

sources of hybrid distributed generation are

wind generator, solar PV, fuel cell, micro-hydro,

small hydro, biomass, geothermal, tides, wave

generator. Wind generator and solar PV are the

most commonly used.

Fig.1. Representation of Distributed generation



III. IMPACT OF TRANSIENT STABILITY ON

DISTRIBUTED GENERATION

As mentioned in the previous section

interconnection of DG to the grid impacts the

stability of the grid to a great extend. In the

earlier DG studies, the impacts on transient

stability were almost neglected since the most

important objective at that point was to

produce electricity from renewable energy.

When the size of DG is quite small, its impacts

on the transient stability will be negligible and

therefore can be neglected. But with the

increase in the number of DG‟s it becomes

essential to analyze the transient stability. It is

only in the recent years that greater focus is

given on the stability impacts due to increasing

usage of DG‟s and their bigger sizes.

In reference [11], the transient stability of

the New England test system is analyzed. The

analysis includes asynchronous generators,

controlled and uncontrolled synchronous

generators as well as controlled and

uncontrolled power electronic converters. In

this approach a fault is applied to the system

for 150 ms and the response of the different

generator technologies with different

penetration levels are monitored.

The response of these generator techniques

are analyzed by means of three indicators

namely rotor speed deviation, oscillation

duration and the terminal voltage. This study

proved that increase in the number of DG‟s

improved the stability of the system irrespective

of the type of technology used. Similar

approach has been used in reference [12], to

analyze the angle, frequency and voltage

stability of a 245-bus system. In this study, the

power angle between two generators, the

deviation in frequency and voltage deviation is

analyzed when a fault of 100ms is applied to

the system. Similar to the previous work, in

this also it was seen that with more DG

percentage, the stability of the system

improves.

IV. MODELING OF HYBRID DISTRIBUTED

GENERATION

There are two methods described in

literature for modelling renewable energy. They

are time-step simulation methods and

probabilistic methods. The time-step

simulation is based on analytical method or

deterministic approach of modelling. It goes

through a simulation period step by step and

the conditions are assumed to be known ahead

of time. Generally, time-step simulation is

deterministic approach where the fault

application or other disturbances are fixed or

set ahead of time. Probabilistic approach on the

other hand is based on a stochastic method. It

is good at processing stochastic uncertainties

such as uncertainties about training data, type

of fault and location of fault coupled with the

conditions of the system. Probabilistic

approach can be used with time-step

simulation. Recently, computational

intelligence (CI) techniques are gaining

recognition in modelling and assessment of

dynamic stability. This research is proposing

the use of computational intelligence method in

modelling and assessment of hybrid DG (HDG).

This is relatively a new idea proposed in this

thesis to determine transient stability margin of

grid integrated HDG instead of using the time-

step simulation or probabilistic method. CI

techniques involve the use of data gathering

and training. CI techniques can be combined

with time-step simulation and also used with

probabilistic method.

This section describes the modelling of wind

generator, solar PV, and small hydropower

system using analytical modelling approach.

The reason for choosing the above three

renewable generators is because they are

available in abundance and environmentally

friendly. Rather than standalone type

distributed generators (DG), the utility

interactive HDG is considered which at the

moment is gaining popularity.

There are two types of models in hybrid power

generation.

1. Logistical models

2. Dynamic models

Logistical models are used primarily for

studying long time performance, economic

analysis, component sizing and prediction

whereas dynamic models are used mainly for

component design, assessment of system

stability and power quality.

Wind generator, solar PV and small hydropower

for distributed generation applications are

explained in the following sections.



V. SMALL HYDRO POWER TURBINE

In a hydropower system, the energy present

in water is converted into mechanical or

electrical energy by the use of hydropower

plant. Generic hydro power systems can be

categorized in many different ways. Some of the

methods of classification are based on how the

electricity is generated by the plant, what kind

of grid system is utilized for the distribution of

electricity, the type of load capacity and the

type of storage used by the system.

Small hydro power plants are designed to

generate electrical or mechanical power

based on the demand for energy of the

surrounding locality. In a typical SHS (Small

Hydro-power System) the water from the source

is diverted by weir through an opening intake

into a canal (Fox, 2004) . A settling basin might

sometimes be used to sediment.

Eign particles from the water. The canal is

designed along the contours of the landscape

available so as to preserve the elevation of the

diverted water. The water then enters the fore-

bay tank and passes through the penstock

pipes which are connected at a lower elevation

level to the turbine. The turning shaft of the

turbine is then used to operate and generate

electricity. The machinery or appliances which

are energized by the hydro scheme are called

the load.The power available in water current is

proportional to the product of head and flow

rate.

The general formula for any hydro power is

(1)

P is the mechanical power produced at the

turbine shaft (Watts), ρ is the density of water

(1000 kg/m3), g is the acceleration due to

gravity (9.81 m/s2), Q is the water flow rate

passing through the turbine (m3/s), H is the

effective pressure head of water across the

turbine (m).

Figure 2 A typical SHS (Small Hydropower System)

VI. SOLAR POWER PLANT

Photovoltaic (PV) power systems convert

sunlight directly into electricity. A residential

PV power system enables a homeowner to

generate some or all of their daily electrical

energy demand on their own roof, exchanging

daytime excess power for future energy needs

(i.e. night time usage). The house remains

connected to the electric utility at all times, so

any power needed above what the solar system

can produce is simply drawn from the utility.

PV systems can also include battery backup or

uninterruptible power supply (UPS) capability

to operate selected circuits in the residence for

hours or days during a utility outage.

In this early stage of marketing solar

electric power systems to the residential

market, it is advisable for an installer to work

with well established firms that have complete,

pre-engineered packaged solutions that

accommodate variations in models, rather than

custom designing custom systems. Once a

system design has been chosen, attention to

installation detail is critically important. Recent

studies have found that 10-20% of new PV

installations have serious installation problems

that will result in significantly decreased

performance. In many of these cases, the

performance shortfalls could have been

eliminated with proper attention to the details

of the installation.

Figure 3 Model for single solar cell

The output terminal of the circuits is

connected to the load. The output current

source is the different between the

photocurrent Iph and the normal diode current

ID. Ideally the relationship between the output

voltage V and the load current I of a PV cell or a

module.



Where Iph is the photocurrent of the PV

cell (in amperes), I0 is the saturation current,

Ipv is the load current (in amperes), Vpv is the

PV output voltage(in volts), Rs is the series

resistance of the PV cell (in ohms) and m, K

and Tc represent respectively the diode quality

constant, Boltzmann‟s constant and

temperature.

PV effect is a basic physical process

through which solar energy is converted

directly into electrical energy. It consists of

many cells connected in series and parallel.

The voltage and current output is a

nonlinear relationship. It is essential

therefore to track the power since the

maximum power output of the PV array varies

with solar radiation or load current. This is

shown by Matlab simulation.

Figure 4 Model Photovoltaic plates

VII. WIND POWER PLANT

Wind energy conversion systems is one of

the challenges to achieve knowledge for the

ongoing change due to the intensification of

using wind energy in nowadays. This book

chapter is an involvement on those models, but

dealing with wind energy conversion systems

consisting of wind turbines with permanent

magnet synchronous generators (PMSG) and

full-power converters. Particularly, the focus is

on models integrating the dynamic of the

system as much as potentially necessary in

order to assert consequences on the operation

of system.

In modelling the energy captured from the

wind by the blades, disturbance imposed by

the asymmetry in the turbine, the vortex tower

interaction, and the mechanical eigen swings in

the blades are introduced in order to assert a

more accurate behaviour of wind energy

conversion systems. The conversion system

dynamic comes up from modelling the dynamic

behaviour due to the main subsystems of this

system: the variable speed wind turbine, the

mechanical drive train, and the PMSG and

power electronic converters. The mechanical

drive train dynamic is considered by three

different model approaches, respectively, one

mass, two-mass or three-mass model

approaches in order to discuss which of the

approaches are more appropriated in detaining

the behaviour of the system. The power

electronic converters are modelled for three

different topologies, respectively, two-level,

multilevel or matrix converters. The

consideration of these topologies is in order to

expose its particular behaviour and advantages

in what regards the total harmonic distortion of

the current injected in the electric network. The

electric network is modelled by a circuit

consisting in a series of a resistance and

inductance with a voltage source, respectively,

considering two hypotheses: without harmonic

distortion or with distortion due to the third

harmonic, in order to show the influence of this

third harmonic in the converter output electric

current. Two types of control strategies are

considered in the dynamic models of this book

chapter, respectively, through the use of

classical control or fractional-order control.

Case studies were written down in order to

emphasize the ability of the models to simulate

new contributions for studies on grid-

connected wind energy conversion systems.

Figure 5 Configuration of a DFIG driven by a wind

turbine

The general relations between wind speed and

aerodynamic torque

(3)

VIII. TRANSIENT ANALYSIS OF THE SYSTEM

Transient stability of a system is defined as

the ability of the system to return back and



remain in its stable operating condition

following a severe disturbance. In order to

analyze the transient stability of the system

certain stability indicators were chosen by

means of which the stability of the system was

accessed. The description of these indicators,

different scenarios that were considered for the

analysis and the procedure to analyze stability

is explained in this section.

7.1 Transient Stability Indicators

In order to investigate the transient stability

of the test system, certain indicators have to be

selected which indicate stability. The indicators

selected depend upon the type of stability that

needs to be monitored. In this analysis the

different stability indicators are selected based

on references [13].

Terminal voltage -It is known that the

voltage magnitude or phase of the system

changes when there is a disturbance in the

system. When a disturbance is applied to the

system, the terminal voltage changes and when

the fault is cleared it returns back to normal. A

system is said to have better voltage stability if

this change in voltage is less. Rotor speed

deviation- It can be defined as the maximum

change in the speed of the rotor of the machine

when a disturbance is applied to the system.

The degree of stability is analyzed based on the

amount of deviation in the speed. The stability

of a system in which the rotor speed deviation

is less is considered better than a system in

which the speed deviation is more.

IX. PROCEDURE FOR ANALYZING TRANSIENT

STABILITY

A step by step has been followed to analyze the

test system.

A fault is applied at the bus where

distributed generators were connected.

Faults were applied for two different

conditions with and without energy storage

device respectively.

The response of the system to the fault is

analyzed by means of the transient stability

indicator that was chosen.

This procedure is repeated for different

types of faults.

The response of the indicators to the fault

is analyzed for the system.

X. RESULTS AND DICCUSSION

For the technical analysis, MATLAB/

Simulink have been used. Simulink has been

selected due to the reason that it is user-

friendly and consists of a number of ready to

use models. The power system components in

the Simulink library include different types of

generators, loads, transformers, faults and

other components, which can be used to build

and analyze the necessary test system. There

are no built-in energy storage models in

Simulink. Though there are no built-in models

of energy storage, it can easily be built using

the basic blocks that are present. The response

of the selected indicators are monitored and

recorded by means of the scope. Analysis is

done both with and without energy storage

device (Lithium-ion). This chapter presents the

results of transient stability analysis with and

without energy storage device. Two different

fault conditions were considered for the

stability analysis.

Discrete,Ts = 5e-005 s.

powergui

V

T

Variable Head Tank

A

B

C

A

B

C

Three-Phase Fault

VabcIabc

A

B

C

abc

A B C

f(x)=0

Solver

Configuration

Pm

E

m

A

B

C

SSM

Scope

C

AS

Rotational

Hydro-Mechanical

Converter

A B

Resistive Pipe LP PS S

PS-Simulink

Converter1

PS S

PS-Simulink

Converter

Mechanical

Rotational Reference

RC

T

Ideal Torque Sensor

Hydraulic Fluid

0.01316

Display15e+004

Display

0.7

Constant

Figure 6 Simulation Hydro Power Plant

Simulation Circuit for Small Hydro Power plant

Figure 6 shows the small hydro power plant. A

SLG fault is created at distribution level from

0.2 to 0.3 seconds.

Fig 7 Voltage and Current waveform (Fault from 0.2 to

0.3 sec)



PCC

solar output


powergui

v+-

Voltage Measurement1

v+-

Voltage Measurement

g

A

B

C

+

-

VSI inverter

Vabc

IabcA

B

C

a

b

c

Three-Phase V-I Measurement

A

B

C

A

B

C

Three-Phase Fault

A B C

Scope1Scope

Pulse GeneratorPulses

PWM Generator

+

-

PV ARRAY

gm

DS

Mosfet

LF2

LF1

LFDiode

i+

-

Current Measurement

Cf2Cf1Cf

Fig 8 Simulation Circuit for Solar Power plant

Figure 8 shows the solar power plant. A SLG

fault is created at distribution level from 0.4 to

0.7 seconds.

Fig 9 Voltage and Current waveform for SLG (Fault

from 0.4 to 0.7 sec)

Fig 9 shows the voltage and current waveform.

A single line to ground fault is created from 0.4

sec to 0.7 sec. Voltage observes a dip in the

faulted phase and current wave observed a rise

in that phase.

Fig 10 Voltage and Current waveform for Three phase

fault (Fault from 0.4 to 0.7 sec)

Fig 9 shows the voltage and current waveform.

A three phase fault is created from 0.4 sec to

0.7 sec. Voltage drops to zero and current rises

to a very high value. Discrete,

Ts = 5e-005 s.

powergui

10

0

output

Generator speed

Pitch angle (deg)

Wind speed (m/s)

Tm (pu)

Wind Turbine1

v+-

Vdc5

v+-

Vdc4Vdc3

v+-

Vdc2

Vdc1

v+ -

Vdc

A

B

C

A

B

C

Three-Phase Fault

Vabc

IabcA

B

C

a

b

c

A B C

Parallel RLC Branch1

Parallel RLC Branch

Pulses

PWM Generator

Tm

mA

B

C

PMSM

gm

DS

Mosfet5

gm

DS

Mosfet4

gm

DS

Mosfet3

gm

DS

Mosfet2

gm

DS

Mosfet1

gm

DS

Mosfet

[F]

Goto5

[E]

Goto4

[D]

Goto3

[C]

Goto2

[B]

Goto1

[A]

Goto

[F]

From5

[E]

From4[C]

From3

[D]

From2[B]

From1

[A]

From

ma

k

Diode5

ma

k

Diode4

ma

k

Diode3

ma

k

Diode2

ma

k

Diode1

ma

k

Diode

<Rotor speed wm (rad/s)>

Fig 11 Simulation Circuit for Wind plant

Figure 11 shows the wind power plant.

Different faults are created at distribution level

from 0.3 to 0.4 seconds.

Fig 12 Voltage and Current waveform for SLG (Fault

from 0.2 to 0.35 sec)

Fig 13 Voltage and Current waveform for three phase

to ground fault (Fault from 0.2 to 0.35 sec)


powergui

g

A

B

C

+

-

A

B

C

A

B

C

A

B

C

Three-Phase Fault

Vabc

Iabc

A B Ca b c

Vabc

IabcA

B

C

a

b

c

A

B

C

a2

b2

c2

a3

b3

c3

A B C

In1Out1

Subsystem1

Scope5

Scope2

Scope1

R

Y

B

SOLAR POWER PLANT

signal rms

RMS

R

Y

B

HYDRO POWER PLANT

[linevoltage]

Goto1

[linevoltage]

From1 abc

Mag

Phase

Discrete 3-phaseSequence Analyzer

UrefPulses

DiscretePWM Generator

PI

DiscretePI Controller

R

Y

B

DFIG WIND GENERATION

1

Constant

Add

Fig 14 Simulation Circuit of Hybrid DG

Fig 15 Voltage waveform for three phase to ground

fault (Fault from 0.2 to 0.3 sec) with STATCOM



XI. CONCLUSION

Modeling and simulation of grid integrated

Hybrid distributed generation is simulated . It

majorly investigates the impact of various

complimentary energy sources on the power

system network. The test system is single

machine infinite system with integrated HDG.

The system was observed by using oscillation

duration and critical clearing time. The final

report shows that, as the number of generator

increases the stress on the system also

increases. However, the simulation shows that

hybrid power system with three generator show

a critical cases compared to two DG. The result

shows that the impact depend on the network

strength, level of penetration and the

technologies involves

I have simulate the hydro wind solar

power plant with alone and hybrid

distributed system. Modeling and simulation is

done with renewable source. The output of

three power plant is couple and simulated.

Hybrid distributed of Hydro Solar and Wind

power plant is connect with AC Grid. The

combination of three plant and outputs are

shown in Mat lab simulation.

REFERENCES

[1] J.Momoh, G.D.Boswell, “Improving power Grid

Efficiency using Distributed Generation”

Proceedings of the 7th international conference

on power system operation and planning, pp.11-

17, Jan., 2007

[2] T.Ackerman,G. Andersson, and L.Soder.

„Distributed generation, a definition‟ Electric

Power Energy Research Vol,57, no3, pp195-

204,2001.

[3] Distributed Generation and Cogeneration Policy

Roadmap for California.

[4] A.M.Azmy, Simulation and Management of

Distributed Generation units using

intelligent techniques, Ph.D. Thesis submitted to

the department electrical engineering

University of Duisburg-Essen 2005.

[5] C.Wang, „Modeling and control of hybrid

wind/photovoltaic/fuel distributed generation

system‟ PhD thesis Submitted to Montana State

University Bozeman,Montana July 2006.

[6] V.V Thong, J. Driesen, R. Belmans,

“Transmission system operation concerns

with high penetration level of distributed

generation”, Universities power and Engineering

Conference UPEC, pp.867-8717, 4-6Sept., 2007.

[7] T. Tran-Quoc, L.Le Thanh, Ch.Andrieu, N.

Hadjsaid, C. Kieny, J.C Sabonnadiere,

K.Le,O.Devaux,O.Chilard „;Stability analysis for

the distribution networks with distributed

generation” in Proc.2005/2006 IEEE/PES

Transmission & Distribution Conference &

Exposition, vol. 1–3, Dallas, TX, USA,

May 21–26 Proceedings Paper, pp. 289–294.

2006.

[8] V.V. Knazkins “Stability of Power Systems with

Large Amounts of Distributed Generation

“PhD Thesis submitted to University Stockholm,

Sweden, 2004.

[9] A Ishchenko „Dynamics and stability of

distributed networks with dispersed Generation,

Ph.D thesis submitted to Eindho-ven University

of technology,16 Jan.,2008.

[10] M. Reza, Stability analysis of transmissions

Systems with high penetration of distributed

generation”, PhD Thesis submitted to Delf

University of Technology Delft, The

Netherlands, 2006.

[11] Y.Tiam, “Impact on the power system with a

large generation of photovoltaic generation”,

PhD Thesis submitted to the University of

Manchester Institute of Science and

Technology, Feb., 2004, pp.53-55.

Documents

Impact of Hybrid Distributed Generator on Transient ...ijmtst.com/documents/NCCEESES15.pdf · is done in a user friendly MATLAB/Simulink environment. ... behind the use of hybrid